In recent years, importance of solar cells has been increasing as an alternative energy to fossil fuel and as a measure against global warming. However, existing solar cells such as silicon solar cells are costly at present, which is a factor of preventing them from being widely used.
Therefore, research and development on various low-cost solar cells have been advancing. In particular, a dye-sensitized solar cell presented by Graetzel, et al. at Swiss Federal Institute of Technology in Lausanne has increasingly been expected to put into practice (see, for example, PTL 1 and NPLs 1 and 2). The structure of this solar cell is composed of a transparent electroconductive glass substrate; a porous metal oxide semiconductor on the substrate; a sensitizing dye adsorbed on a surface of the semiconductor; an electrolyte containing a redox pair; and a counter electrode. Graetzel, et al. remarkably improved photoelectric conversion efficiency by making porous a metal oxide semiconductor electrode such as titanium oxide to enlarge its surface area and by adsorbing a ruthenium complex as a dye in a monomolecular manner.
However, ruthenium used in the sensitizing dye is a rare metal and has limitation on its amount. Under such circumstances, development has been required for organic dyes having no limitation on their amounts. Sensitizing dyes already proposed include xanthene dyes such as Eosin Y (see NPL 3), perylene dyes (see NPL 4), cyanine dyes (see PTL 2), merocyanine dyes (see PTL 3), coumarin dyes (see NPL 5), polyene dyes (see NPL 6), porphyrin dyes (see NPL 7), and phthalocyanine dyes (see NPL 8).
Xanthene dyes such as Eosin Y and perylene dyes have an absorption wavelength range of up to about 600 nm only, and disadvantageously are low in conversion efficiency. Cyanine dyes, merocyanine dyes, polyene dyes, coumarin dyes, and other dyes have many conjugated double bonds, and hence tend to cause cis-trans isomerization due to light irradiation and disadvantageously are quite low in durability. Porphyrin dyes and phthalocyanine dyes are disadvantageously low in solubility and adsorption intensity onto titanium oxide, and are easily detached from titanium oxide. Phthalocyanine dyes also have difficulty in adapting to titanium oxide in terms of energy level.
Recent reports presented that push-pull porphyrin complexes containing a hole transporting compound (see NPL 9) have quite high conversion efficiency. However, their absorption wavelength range is about 750 nm, and there has been a need to achieve higher conversion efficiency resulting from absorption in longer wavelength.
As described above, at present, none of the sensitizing dyes for dye-sensitized solar cells that have been considered so far has satisfactory properties.